Organic electroluminescent devices (OLED) are highly valued in flat panel display and lighting fields due to the advantages of high brightness, color saturation, thin and light, flexible and so on. At present, a common organic electroluminescent device mainly comprises top-emitting device structure and bottom-emitting device structure. Compared with the bottom-emitting device structure, the top-emitting device structure has advantages of high aperture ratio, high color purity, easy to realize high PPI and so on. Therefore, the top-emitting device structure has become the mainstream of organic electroluminescent device structure. However, there are some problems in the structure of top-emitting organic electroluminescent devices, such as low efficiency, high driving voltage, poor lifetime, viewing angle and so on.
The existing organic electroluminescent device structure typically comprises an anode layer, a hole transport layer (HTL, hole transport layer), a light emitting layer (EML, emissive layer), an electron transport layer (ETL, electron transport layer), and a cathode layer (as shown in FIG. 1). The working principle of the device is: holes are injected from the anode, and transported on the HOMO energy level (highest occupied molecular orbital) of the hole transport layer, eventually reaching the HOMO energy level of the light emitting layer; electrons are injected from the cathode, and transported on the LUMO energy level (lowest unoccupied molecular orbital) of the electron transport layer, eventually reaching the LUMO energy level of the light emitting layer. Under the action of applied voltage, the electrons on the LUMO energy level of the light emitting layer are combined with the holes on the HOMO energy level of the light emitting layer, so as to realize light emitting of the organic electroluminescent device. Typically, an optimal device structure should realize a balanced injection for electrons and holes, such that each of the electrons and holes is recombined, so as to obtain a device structure with high efficiency. However, it is difficult to obtain a light emitting material, of which the electron mobility and hole mobility are the same.
Due to the inconsistency between the electron-transport ability and hole-transport ability in the light emitting layer, the electrons and holes transported to the interface of the light emitting layer can not enter the light emitting layer and recombine with a same number, which is not beneficial for improving the efficiency and lifetime of the organic electroluminescent device.